1. Field of the Invention
The present invention relates to a computer program product, system, and method for establishing reverse paths between servers in a copy environment.
2. Description of the Related Art
In a storage environment, a storage controller may maintain mirror copy relationships, where a source volume in a mirror copy relationship comprises the storage or volumes from which data is physically copied to a target volume. Failover programs, such as International Business Machines Corporation's (“IBM”) HyperSwap® which is a function in the z/OS® operating system, provides continuous availability for disk failures by maintaining the mirror copy relationships to provide synchronous copies of source (primary) disk volumes in one or more storage systems to one or more target (secondary) volumes in one or more storage systems. (HyperSwap is a registered trademark of IBM in countries throughout the world). When a disk failure is detected, code in the operating system identifies HyperSwap managed volumes and instead of failing the I/O request, HyperSwap switches (or swaps) information in internal control blocks so that the I/O request is driven against the target volume of the mirror copy relationship. Since the target volume is an identical copy of the source volume prior to the failure, the I/O request will succeed with no impact to the program issuing the I/O request, which could be an application program or part of the operating system. This therefore masks the disk failure from the program and avoids an application and/or system outage.
A mirror copy relationship may maintain a current and previous bitmaps to keep track of updates at the source volume that need to be copied or synchronized to the target storage. A previous bitmap, also known as an out-of-synch bitmap, indicates updated data in the source volume that occurred in a previous interval, or consistency period, and a current bitmap, also known as a change recording bitmap, which indicates updated data in the source volume that occurred in the current interval or current consistency period. After the replication manager copies all updated data indicated in the previous bitmap, the bitmaps would be toggled to create a new interval, so that the previous bitmap is set to the current bitmap to copy all updated data prior to the new interval, and a new current bitmap would be initialized to record writes in the new interval. In this way, updates that occur while data is being synchronized get recorded without interfering with the synchronization of the writes as of the recent interval.
A bitmap toggle performed out-of-sequence could cause out-of-sync indicators to be reset when they should not be compromising the integrity of data in a remote copy relationship. A node sending a toggle message waits for responses from the nodes it sent messages to. If a response is not received within some amount of time, the messages must be aborted. Old messages are aborted by updating a sequence number. Once toggle messages are responded to or aborted by a sender-receiver pair handshaking a new sequence number, subsequent toggle messages can be sent.
One technique to allow a target to send a source server a toggle or other message is to have the source server send a read command to the target server and when the target has data or a message, the target responds to the read with the message or data to return.
There is a need in the art to provide improved messaging between target servers and source servers in a cascaded replication environment of numerous source and target pairs that are linked together.